1. Technical Field
The present invention relates to a process for drilling into a subterranean formation for the recovery of hydrocarbons, and more particularly to a drilling process wherein a foamed gel is employed as a drilling fluid, a lost circulation fluid, or a combined drilling/lost circulation fluid.
2. Description of Related Art
A drilling fluid is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The circulation path of the drilling fluid typically extends from the surface wellhead down through the drill pipe string to the drilling face and back up through the annular space between the drill pipe string and wellbore face to the surface wellhead. The drilling fluid performs a number of functions as it circulates through the wellbore including cooling and lubricating the drill bit, removing drill cuttings from the wellbore, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts.
There are a number of well-known conventional drilling fluids including compositions termed "drilling muds". Drilling muds comprise high-density dispersions of fine solids in an aqueous liquid or a hydrocarbon liquid. An exemplary drilling mud is a dispersion of clay and/or gypsum in water. The solid component of such a dispersion is termed a "weighting agent" and is designed to enhance the functional performance of the drilling fluid. Other conventional drilling fluids include air, mists consisting of liquid particlulates dispersed within a gas medium, and conventional foams consisting of a gas contained within an aqueous liquid medium.
Although conventional drilling fluids perform satisfactorily in many subterranean applications, high-density drilling fluids such as drilling muds are generally unsuitable where the hydrostatic pressure gradient of the drilling fluid is greater than the fracture or parting pressure gradient of the rock surrounding the wellbore. Conventional drilling fluids may also be unsuitable in water-sensitive formations because of the risk of formation damage due to incompatibilities between the drilling fluid and the formation. Conventional drilling fluids may likewise perform unsatisfactorily in formations that are susceptible to lost circulation. Lost circulation is the migration of drilling fluid from the wellbore across the wellbore face into or out of the surrounding formations resulting in excessive loss of drilling fluid. Lost circulation is undesirable because it requires replacement of the lost drilling fluid, thereby adding a significant cost to the drilling operation. Lost circulation can also damage the hydrocarbon-producing zone or result in a well blowout with serious safety and environmental consequences.
In response to the problem of lost circulation, it is common to place a fluid in the wellbore containing additives termed "lost circulation materials" that specifically inhibit fluid communication between the wellbore and surrounding formations across the wellbore face. Lost circulation materials are frequently polymeric species as described in U.S. Pat. Nos. 4,740,319; 4,726,906; 4,675,119; and 4,282,928. A liquid medium having a lost circulation material dissolved or dispersed therein is termed a lost circulation fluid. Despite the general effectiveness of many conventional lost circulation fluids, certain subterranean conditions remain problematic for such fluids. In particular, conventional lost circulation fluids often do not effectively inhibit lost circulation in formations having relatively high permeability matrix or relatively high conductivity anomalies. Conventional lost circulation fluids may also be inapplicable in water-sensitive formations or formations susceptible to fracturing or parting.
Crosslinked polymer gels as taught by U.S. Pat. No. 4,989,673 have demonstrated performance advantages over the above-recited conventional drilling fluids and lost circulation fluids, because in many instances the gels effectively inhibit fluid loss in formations having high permeability matrix or high conductivity anomalies, while generally avoiding significant damage to water-sensitive formations. The relatively high chemical cost of crosslinked polymer gels, however, often limits their practical utility from an economic standpoint. Crosslinked polymer gels also have a relatively high hydrostatic pressure gradient in the wellbore that is undesirable for formations susceptible to fracturing or parting by conventional fluids of normal density.
Conventional foams have been employed as alternative drilling fluids or lost circulation fluids in formations susceptible to fracturing because such foams have a relatively low density and correspondingly low hydrostatic pressure gradient in the wellbore. Nevertheless, conventional foams have a number of shortcomings that limit their utility in wellbore drilling operations. Conventional foams often lack sufficient structure to effectively suspend and transport rock cuttings to the surface. In many cases, conventional foams also lack sufficient structure to adequately stop fluid loss, tending only to reduce the rate of fluid loss rather than stopping fluid loss altogether. Conventional foams also exhibit relatively high instability under certain formation conditions. For example, conventional foams tend to exhibit instability in the presence of crude oil. In addition, conventional foams often degrade when placed in formations having high downhole temperatures or in formations having brines exhibiting a high salt or hardness content.
Accordingly, it is an object of the present invention to provide a drilling fluid that performs effectively in conventional drilling operations. It is a particular object of the present invention to provide a drilling fluid that effectively suspends and transports rock cuttings to the surface during a drilling operation.
It is another object of the present invention to provide a lost circulation fluid that effectively prevents lost circulation in a wellbore under a broad range of subterranean conditions. It is yet another object of the present invention to provide a single fluid that performs as a combined drilling/lost circulation fluid. It is still another object of the present invention to provide a lost circulation material that can be added to a drilling fluid to form a combined drilling/lost circulation fluid that effectively prevents lost circulation of drilling fluid under a broad range of subterranean conditions. It is a particular object of the present invention to provide a lost circulation fluid or combined drilling/lost circulation fluid that effectively prevents lost circulation in a subterranean formation exhibiting relatively high permeability or high conductivity anomalies. It is yet another object of the present invention to provide a drilling fluid, lost circulation fluid or drilling/lost circulation fluid that effectively performs in a subterranean formation having a relatively low fracture or parting pressure gradient without substantially fracturing or parting the formation. It is still another object of the present invention to provide a drilling fluid, lost circulation fluid or drilling/lost circulation fluid that is relatively stable under harsh formation conditions including the presence of high temperatures, crude oil, high salinity brines or high hardness brines. It is further an object of the present invention to provide such a drilling fluid, lost circulation fluid or drilling/lost circulation fluid that is cost effective, having practical economic utility in the field.